The story is about a young orphaned girl named Sophie, living in a girl's orphanage run by the cantankerous and abusive Mrs. Clonkers. One night, Sophie sees a large, cloaked person blowing something via a trumpet-like object into a bedroom window down the street. She is discovered by the mysterious person, who carries her to his homeland of Giant Country.

There, he identifies himself as the Big Friendly Giant ('BFG'), who nightly blows bottled dreams into the bedrooms of children, and explains the other type of giants that eat humans, mostly children. Because the BFG refuses to eat people or steal food from humans, he subsists on a foul-tasting vegetable known as a snozzcumber.It's like a cucumber with black strips.

Sophie and the BFG quickly become friends; but Sophie is soon put in danger by the sudden arrival of the Bloodbottler Giant, who suspects the BFG of harboring Sophie. Sophie hides in the snozzcumber, unknown to the BFG, and the BFG offers the snozzcumber to the Bloodbottler, hoping that its foul taste will repel him from the area; whereupon the Bloodbottler spits out the snozzcumber and Sophie, and leaves in disgust. When Sophie announces she is thirsty, the BFG treats her to a fizzy drink called frobscottle, which causes noisy flatulence because of the bubbles sinking downwards. The BFG calls this "Whizpopping". The next morning, the BFG takes Sophie to Dream Country to catch more dreams, but is tormented by the other giants along the way; notably by their leader, the Fleshlumpeater, the largest and most fearsome.

In Dream Country, the BFG demonstrates his dream-catching skills to Sophie; but the BFG mistakenly captures a nightmare, and uses it to start a fight among the other giants when Fleshlumpeater has a nightmare about Jack. Sophie later persuades him to approach the Queen of England toward imprisoning the other giants. To this end, she uses her knowledge of London to navigate the BFG to Buckingham Palace, and the BFG creates a nightmare, introducing knowledge of the man-eating giants to the Queen, and leaves Sophie in the Queen's bedroom to confirm it. Because the dream included the knowledge of Sophie's presence, the Queen believes her and speaks with the BFG.

After considerable effort by the palace staff to create a table, chair, and cutlery of appropriate size, the BFG is given a delicious breakfast, and the Queen telephones the King of Sweden and the Sultan of Baghdad to confirm the BFG's story – the giants having visited those locations on the previous two nights – then summons the Head of the British Army and the Marshal of the Royal Air Force. The said officers, though initially belligerent and skeptical, eventually agree to cooperate.

A fleet of helicopters then follows Sophie and the BFG to the giants' homeland, where the giants are tied up as they sleep, suspended under the helicopters, and carried back to London, where they are imprisoned in a pit. The only one not easily caught is the Fleshlumpeater, who wakes up as the British attempt to tie him up, but Sophie and the BFG trick him into allowing his own capture by claiming that he has been poisoned by a venomous snake so that he will put his hands and feet together to be tied up. The man-eating giants are then imprisoned in a deep pit where they are only fed snozzcumbers.

Afterwards, a huge castle is built as the BFG's new house, with a little cottage next door for Sophie. While they are living happily in England, with several gifts coming in for many years from the governments of every country ever targeted by the giants (notably England, Sweden, Iraq, Arabia, India, Panama, Tibet, the United States, Chile, Jersey and New Zealand), the BFG writes a book of their adventures, which is then identified as the novel itself.

Sophie: The imaginative and kind-hearted protagonist of the story who becomes a brave international heroine. Portrayed by Amanda Root in the 1989 film, and Ruby Barnhill in the 2016 film.

The BFG: A friendly, benevolent, gentle, sweet 24-foot-tall giant who has superhuman hearing abilities and immense speed. His primary occupation is the collection and distribution of good dreams to children. He also appears in another novel, Danny, the Champion of the World, in which he is introduced as a folkloric character. His name is an initialism of 'Big Friendly Giant.' **Portrayed by David Jason in the 1989 film and Mark Rylance in the 2016 film.

Mrs. Clonkers: The unseen director of the orphanage in which Sophie lives at the start of the novel; described as cruel to her charges. Portrayed by Myfanwy Talog in the 1989 film and by Marilyn Norry in the 2016 film,.

Nine Man-Eating Giants: Each one is about 50-feet-tall and proportionately broad and powerful. According to the BFG about the flavors of the humans that the man-eating giants dine on, the Turkish tasting like turkey, the Greeks are too greasy, people from Panama taste like hats, the Welsh taste fishy, the people from Jersey taste like cardigans, and the Danes taste like dogs.

The Fleshlumpeater: The leader of the nine man-eating giants and the most horrible of the bunch. Portrayed by Don Henderson in the 1989 film and by Jemaine Clement in the 2016 film.

The Bloodbottler: Second in command to the Fleshlumpeater who has a fondness for the taste of human blood. Portrayed by Don Henderson in the 1989 film and by Bill Hader in the 2016 film.

The Manhugger: One of the nine man-eating giants. Portrayed by Adam Godley in the 2016 film.

The Meatdripper: One of the nine man-eating giants. He pretends to be a tree in a park so that he can pick off the humans that go under him. Portrayed by Paul Moniz de Sa in the 2016 film.

The Childchewer: One of the nine man-eating giants. Portrayed by Jonathan Holmes in the 2016 film.

The Butcher Boy: One of the nine man-eating giants. Portrayed by Michael Adamthwaite in the 2016 film.

The Maidmasher: One of the nine man-eating giants. Portrayed by Ólafur Ólafsson in the 2016 film.

The Bonecruncher: One of the nine man-eating giants who is known for crunching up two humans for dinner every night. He enjoys eating people from Turkey making him the picky eater of the bunch. Portrayed by Daniel Bacon in the 2016 film.

The Gizzardgulper: One of the nine man-eating giants. He lies above the rooftops of the cities to grab people walking down the streets. Portrayed by Chris Gibbs in the 2016 film.

In 2012 it was ranked number 88 among all-time children's novels in a survey published by School Library Journal, a monthly with primarily U.S. audience. It was the fourth of four books by Dahl among the Top 100, more than any other writer.[8]